In the last few months two more
binary systems have been discovered. Astronomers M. E.
Brown and J. L. Margot, at California Institute of
Technology, reported the discovery of a satellite to
asteroid (87) Sylvia. The images were taken on Feb. 18,
2001 using the adaptive optics system on the 10-meter
Keck II telescope in Hawaii. The brightness ratio between
(87) Sylvia and its moon was measured to be of the order
of 450, suggesting a ratio of radii of about 20:1. It is
also estimated that the moon completes an orbit once
every four days.The latest discovery of a binary
asteroid was reported by astronomer C. Veillet, at
Canada-France-Hawaii Telescope (CFHT), on Apr. 16, 2001
(see IAUC 7610). The recovery images of the transneptunian
object TNO (dict.) 1998 WW_31, taken by Veillet, A.
Doressoundiram and J. Shapiro with the 3.6-meter CHFT,
showed the presence of two close, but distinguishable
objects moving together over the two nights of
observation (Dec. 22 and 23, 2000). With the help of
archival images of 1998 WW_31, taken nearly a year
before, again at CHFT, it was possible to secure its
binary nature. Hence, 1998 WW_31 is the second TNO (after
Pluto) recognized to have a satellite. While the ratio of
radii of the pair Pluto/Charon is 2:1, 1998 WW_31 is
believed to be essentially a double object with
comparable components: a preliminary analysis of Dec. 22,
2000 image shows that the two component differ of only
0.4 magnitudes (dict.)
(R band) between each other.

It is now well-established that mutual
collisions have played and still play a fundamental role in the
evolution of the whole asteroidal populations. During the first
stages of the evolution of the Solar System, the debris
constituting the primordial disk collide at very low relative
velocity, allowing the accretion of the first small planetary
bodies, called "planetesimals". Nowadays due to higher
eccentricities and inclinations as a result of their dynamical
evolution, asteroids collide at mean relative velocity from 5 up
to 20 kilometers per second and the energies involved in these
processes are usually higher than the energy needed to escape the
gravitational well of such bodies. For this reason, the current
phase of the collisional evolution is actually destructive.

Of the two colliding bodies, the smaller
one, which is usually called "projectile", is
completely shattered during the impact, while the bigger body,
called "target", can undergo a wide spectrum of
mechanical modifications: from surface craterization to complete
shattering and dispersion. Essentially it depends on a few
factors, such as the relative kinetic energy between the
projectile and the target, the strength of the material which
constitutes them, the internal structure and the target
self-gravitational potential energy. Craterization is the most likely process.
Indeed, it is the process of which we have the widest direct
proof: every planet and asteroid keeps evident tracks of this
kind of event. However, it must be said that almost all known
asteroids, maybe with the exception of the biggest ones,
are"fragments" of catastrophic collisions. For example,
the Near Earth Asteroid 433 Eros, recently approached and studied
by the NASA probe NEAR, is surely a huge impact rocky chunk.The transition between craterization and
catastrophic disruption (conventionally defined as the phase in
which the mass of the impact largest remnant is less than half
the mass of the target) is everything but sharp. What could
happen is that the result of a collision can dangerously approach
this limit without exceeding it. For example, asteroid 253
Mathilde shows an impressive impact crater (nearly 30 kilometers
wide), which is comparable with the dimensions of the asteroid
itself. Another remarkable example is crater Stickney on Phobos,
the larger and innermost of the two moons of Mars, believed to be
an asteroid captured by the Red Planet. It is not difficult to
imagine that in these cases the energy released by the impact
could have led to the total disruption of these bodies. Their
internal structure (maybe porous aggregate) has probably damped
the propagation of impact shock wave, sheltering the most distant
regions of the target from disruption and dispersion. When the impact energy exceeds the
craterization limit, the target experiences a catastrophic
break-up. In the case in which almost all the fragments have a
velocity greater than the target escape velocity (dict.), they
run away giving birth the to what is called an Asteroidal Family.
It is possible that during the escape process two or more
fragments have a relative velocity lower than the mutual escape
velocity. In this case they could form binary asteroids or
re-accumulated bodies, usually called "rubble-piles".
Binary asteroids or rubble-piles can be also generated when the
target is shattered but not completely dispersed, since some
fragments cannot escape the target self-gravity. In the latter
case this ensemble of fragments re-accumulates and, during the
collapse, it begins to rotate faster (according to the
conservation law of the total
angular momentum (dict.), the initial one plus that imparted
by the projectile) reaching a rotational equilibrium shape
similar to those showed by a fluid in hydrostatic rotational
equilibrium. They are, in order of increasing initial angular
momentum, bi-axial ellipsoids (or Maclaurin spheroids), tri-axial
ellipsoids (or Jacobi ellipsoids) and the balanced (equal masses)
binary systems (or Darwin ellipsoids). Recently radar-resolved Near Earth
Asteroids 4179 Toutatis and 4769 Castalia are found to have very
unusual shapes and they are probably contact binaries with
monolithic lobes or even rubble-pile binaries, namely
rubble-piles rotating so fast that their equilibrium
configuration turns out to be a binary (i.e.contact Darwin
ellipsoids).

Asteroid Ida and its
satellite Dactyl - The asteroid Ida was imaged by the
Galileo spacecraft on August 28th 1993.
Image courtesy of NASA

Another well-known binary asteroid is
243 Ida-Dactyl. If we look at the sizes of its components we can
see that this binary system is highly unbalanced: Ida is 58
kilometers long and 23 kilometers wide, while its satellite
Dactyl is approximately egg-shaped, measuring about 1.2 x 1.4
x1.6 kilometers. Although Ida has a highly irregular shape it is
believedto be a monolithic body, not a binary nor a rubble-pile.
It is a member of the Koronis asteroidal family. The origin of
the Ida-Dactyl binary system is probably different from the
scenarios described above. Dactyl might have been originated from
the re-accumulation of a ring of debris around Ida. This debris
could have been injected in orbit as a consequence of a big
impact of a rather massive asteroid on Ida. Its shape (quite
regular, too regular to be a monolithic rocky body of such small
dimensions), its surface spectrum (somewhat different from that
of Ida, suggesting a younger age) and its short dynamical
lifetime (surely shorter than the age of Ida) all give some
support to this formation mechanism.